1. Field of the Invention
The present invention relates to an organic light emitting device and, more particularly, to a stacked active matrix organic light emitting device.
2. Description of the Related Art
Generally, an active matrix organic light emitting device has a plurality of pixels, each of which comprises an electroluminescent (EL) element and a thin film transistor that drives the EL element. In the EL element, red, green, and blue (R, G, and B) organic emission layers are interposed between an anode electrode and a cathode electrode to emit light from the respective organic emission layers based on the voltage applied to the anode electrode and the cathode electrode. The anode electrode is formed such that each R, G and B unit pixel is separated, and the cathode electrode is formed on the entire surface of the substrate.
Stacked organic light emitting devices (SOLEDs), in which a plurality of organic emission layers are stacked, have been described in the art. In SOLEDs, luminous efficiency per unit area can be enhanced by overlapping the organic emission layers within a pixel area.
The conventional SOLED has a structure in which an emission layer is provided n top of on an anode electrode and a cathode electrode is provided on top of on the emission layer. The anode electrode, the emission layer and the cathode electrode are typically sequentially repeated, such that there are several anode-emission layer-cathode groups stacked s on top of one another in the SOLED. A predetermined voltage is applied between the uppermost cathode electrode and the lowermost anode electrode. Because of the voltage applied between the uppermost cathode and the lowermost anode, a vertical current path is formed from the uppermost cathode to the lowermost anode such that current passes through all of the layers. Since light is emitted from several layers stacked on the substrate, the luminous efficiency per unit area is enhanced compared to the conventional organic light emitting device having a single emission layer.
However, in the conventional SOLED, the voltage applied between the uppermost cathode and the lowermost anode to form a single current path in a vertical direction is more than twice the voltage that would be applied in a display device having a single emission layer. Moreover, because a relatively high voltage is used, power consumption increases, and the characteristics of the thin film transistor that drives the EL element are degraded.